Multiple optical input inspection system

ABSTRACT

A system and method of inspecting electrical circuits with multiple optical inputs, including: obtaining first and second image data that are generally spatially coincidental but which each include some image data that is different, modifying one of the images by employing the other image so as to produce an enhanced representation of the electrical circuit, and inspecting the enhanced representation for defects.

FIELD OF THE INVENTION

[0001] The present invention relates to automated optical inspection ofelectrical circuits generally and in particular to methods and apparatusfor generating improved representations of electrical circuits for usein the inspection thereof.

BACKGROUND OF THE INVENTION

[0002] Various types of devices for automated optical inspection ofelectrical circuits are known. Typically one or more gray level imagesof an electrical circuit under inspection are acquired. In someconventional devices for automated optical inspection of electricalcircuits a binary representation of the electrical circuit, generatedfrom a gray level image of the electrical circuit, is employed for atleast some inspection operations. In some automated optical inspectionapplications the binary representation of an electrical circuit underinspection has a spatial resolution which is higher than the spatialresolution of the gray level image.

[0003] The PC Micro II™ and Inspire™ 9060 automated optical inspectionsystems, available from Orbotech Ltd. of Israel, are representative ofconventional automated optical inspection systems for inspectingelectrical circuits. In these conventional systems a gray level image ofan electrical circuit under inspection is acquired. In a first channelthe gray level image is convolved with a function approximating theLaplacian of a Gaussian function. In a second channel a threshold isapplied to pixels in the same gray level image to determine which pixelsin the image are representative of either conductor or substrate, to ahigh degree of confidence. The output of the second channel is appliedto the output of the convolved image from the first channel to modifythe convolved image. The locations of zero-crossings between oppositelysigned pixels in the modified convolved image are calculated, and thezero-crossings subsequently are employed to generate an improvedresolution binary image of an electrical circuit being inspected.

[0004] In other conventional devices for automated optical inspection ofelectrical circuits a contour representation of the electrical circuit,generated from the gray level image of the electrical circuit isemployed for at least some inspection operations. Contours are anapproximation of the location of the transition between regionsexhibiting optically distinguishable characteristics, for examplebetween conductor and substrate in an electrical circuit.

[0005] Additionally, color image acquisition systems recently have beenemployed in the automated optical inspection of electrical circuits.

[0006] The following reference describes edge detection methods:

[0007] D. Marr and E. Hildreth, Theory of Edge Detection, Proceedings ofthe Royal Society of London.

[0008] The following references describe color image processing methods:

[0009] M. Chapron, “A New Chromatic Edge-Detector Used for Color ImageSegmentation”, 11th APR International Conference on Pattern Recognition,Vol. III. IEEE -Computer Society Press, Los Alamitos, Calif., USA, 1992.

[0010] Philippe Pujas and Marie-Jose Aldon, “Robust Colour ImageSegmentation”, 7th International Conference on Advanced Robotics, SanFiliu de Guixols, Spain, Sep. 22, 1995, and

[0011] Leila Shararenko, Maria Petrou, and Josef Kittler, “AutomaticWatershed Segmentation of Randomly Textured Colour Images, IEEETransactions on Image Processing, Vol. 6, no. 11,1997.

[0012] The following U.S. patent application and published PCT patentapplication describe -color image processing methods:

[0013] U.S. Pat. No. 5,483,603 and WO 00/11454

[0014] The following U.S. patents and published PCT patent applicationdescribe techniques employed in automated optical inspection ofelectrical circuits:

[0015] U.S. Pat. No. 5,774,572, U.S. Pat. No. 5,774,573, U.S. Pat. No.5,586,058, U.S. Pat. No. 5,619,429, WO 00/19372 and U.S. Pat. No.6,175,645.

SUMMARY OF THE INVENTION

[0016] The present invention seeks to provide improved techniques andapparatus for automated optical inspection of electrical circuits.Additionally, the present invention seeks to provide apparatus andmethods for generating improved representations of electrical circuits,for example contour representations and binary representations ofelectrical circuits, which may be employed in the automated opticalinspection thereof.

[0017] A general aspect of the present invention relates to methods forgenerating a representation of an electrical circuit having enhancedcontrast between selected portions therein. One implementation of theinvention relates to generating an enhanced contrast representation ofan electrical circuit formed on a non-opaque substrate, however themethod may be employed wherever it is necessary to enhance contrastbetween portions of an electrical circuit portions in an image of theelectrical circuit.

[0018] Preferably the method for generating a representation of anelectrical circuit having enhanced contrast includes evaluating variousportions of multiple optical inputs, such as a digital image, of anelectrical circuit and, in accordance with predetermined logic,enhancing contrast by: i) selectively allowing some portions of thedigital image to retain optical intensity values appearing in the image,and ii) assigning synthetic values to other portions of the digitalimage. Preferably, the assignment of synthetic values to enhancecontrast is performed in a non-linear fashion. For example, theenhancement of contrast may include allowing only portions in the imagethat correspond to conductors located on the top side of a substrate andnon-opaque substrate overlaying conductors on a bottom side thereof toretain an optical intensity value appearing in the digital image, andassigning a synthetic value that is characteristic of substrateoverlaying conductors to all of the portions in the image whichcorrespond to non-opaque substrate.

[0019] A general aspect of the present invention relates to a techniquefor generating an enhanced representation of an electrical circuit usingmultiple optical inputs. The multiple optical inputs may be, forexample, a combination of one or more red, green and blue image inputs.A representation of the electrical circuit is generated using firstimage inputs. Other image inputs, containing information not in thefirst image inputs, are employed during generation of a representationof an electrical circuit being inspected to modify the representationand provide an enhanced output representing the electrical circuit. Theenhanced output representing the electrical circuit typically is used ina conventional manner to inspect the electrical circuit for defects.

[0020] In accordance with a preferred embodiment of the invention adigital image of an electrical circuit is acquired and convolved with afunction to produce a pixelized convolution map of the electricalcircuit. A representation of the electrical circuit derived from imageinputs including information not in the digital image, is employed tooverride at least some pixel values in the convolution map. Theresulting revised convolution map is employed to generate an enhancedrepresentation of the electrical circuit. The enhanced representationmay be, for example, a representation of contours associated withconductors on one side of the electrical circuit, or a binaryrepresentation of conductors located on one side of the electricalcircuit.

[0021] Further in accordance with a preferred embodiment of the presentinvention the digital image is a red image, and the multiple imageinputs are manipulated in a non-linear fashion to produce a “pseudogray” representation of the electrical circuit having enhanced contrastbetween some, but not necessarily all, parts in the electrical circuit.At least some pixel values in the convolved red image are overridden bythe pseudo gray representation to produce a revised convolution image.The revised convolution image is used to calculate an approximatesub-pixel location of transitions between regions having distinguishableoptical characteristics. The approximate sub-pixel locations oftransitions are employed to generate a contour representation or abinary representation of the electrical circuit.

[0022] There is thus provided in accordance with a preferred embodimentof the present invention a method of inspecting electrical circuitscomprising: obtaining first image data relating to at least a part of anelectrical circuit; obtaining second image data generally correspondingto the same part of the electrical circuit, wherein the second imagedata includes at least some image data that is different from the firstimage data; modifying the first image data by employing the second imagedata thereby to produce an enhanced representation of the electricalcircuit; and inspecting the enhanced representation for defects in theelectrical circuit.

[0023] Preferred embodiments of the invention include the preceding oneor more of the following:

[0024] The first image data is in a first spectral range and the secondimage data includes at least some image data in a second spectral range.

[0025] Contrast is enhanced between at least some portions of the secondimage data, wherein the portions represent corresponding parts of theelectrical circuit.

[0026] The contrast enhancing is non-linear.

[0027] The contrast enhancing includes redefining substrate portionswhich, in the second image data, do not overlay conductors as opaquesubstrate portions, thereby to generally eliminate any distinctionbetween substrate portions which overlay conductors and substrateportions which do not.

[0028] The first image data is convolved with a function. Preferably thefunction is an approximation of a Laplacian of a Gaussian function, andthe modifying the first image data is carried out following theconvolving.

[0029] Determining approximate locations of transitions between imageregions having distinguishable optical characteristics in the firstimage data, and removing undesired transitions in response to the secondimage.

[0030] The enhanced representation is a binary representation of theelectrical circuit.

[0031] The enhanced representation is a representation of contours inthe electrical circuit, wherein the contours indicate approximatelocations of transitions between regions in the electrical circuit whichexhibit distinguishable optical characteristics.

[0032] The enhanced representation has a spatial resolution that isgreater than the spatial resolution of the first and second image data.

[0033] The enhanced representation has a gray scale whose dynamic rangeis less than the dynamic range of a gray scale of either the first orthe second image data.

[0034] Determining in the first image data approximate locations oftransitions between image regions having distinguishable opticalcharacteristics, and overriding at least part of the convolved firstimage data.

[0035] The first and second images are acquired with at least oneimager. Preferably the imager comprises at least two different types ofoptical detectors which are arranged to view at least a portion of theelectrical circuit illuminated by the illuminator.

[0036] The first and second images are generally, but not necessarilyexactly, spatially coincidental, and each of the first and second imagesis in a different spectral range.

[0037] There is thus provided in accordance with a preferred embodimentof the present invention a method of inspecting electrical circuitscomprising: obtaining first image data relating to at least part of anelectrical circuit in at least a first spectral range; obtaining secondimage data relating to at least part of an electrical circuit in atleast a second spectral range; and providing an enhanced contrastrepresentation of the electrical circuit by non-linearly combining thefirst image data and the second image data.

[0038] Preferred embodiments of the invention include the preceding andone or more of the following:

[0039] The part of the electrical circuit in the first and second imagesincludes first conductors located on a first side of an electricalcircuit substrate and second conductors located on a second side of anelectrical circuit substrate. Preferably the enhanced contrastrepresentation includes information providing enhanced contrast betweenrepresentations of the first conductors and the electrical circuitsubstrate.

[0040] The enhanced contrast representation exhibits decreased artifactsresulting from a non-opaque characteristic of a substrate.

[0041] There is thus provided in accordance with another preferredembodiment of the present invention a method of inspecting electricalcircuits formed on different surfaces of a non-opaque substratecomprising: obtaining image data relating to at least part of anelectrical circuit, and enhancing the image data to provide enhancedinspection output information which decreases artifacts resulting fromthe non-opaque characteristic of the substrate.

[0042] Preferred embodiments of the invention include the preceding andone or more of the following:

[0043] The electrical circuits include first conductors on a first sideof the substrate and second conductors on a second side of thesubstrate, and the artifacts include part of an image of a substrateportion which does not have either first or second conductors depositedthereon.

[0044] There is thus provided in accordance with a preferred embodimentof the present invention a method of inspecting electrical circuitscomprising: obtaining first image data relating to at least part of anelectrical circuit; obtaining second image data relating to at leastpart of an electrical circuit; and non-linearly combining the firstimage data and the second image data to form an enhanced image of theelectrical circuit.

[0045] Preferred embodiments of the invention include the preceding andone or more of the following:

[0046] The non-linear combining provides a pseudo image

[0047] The pseudo-image is supplied to a high-sure/low-sure regionclassifier operative to classify portions of the image as regions thatto a high degree of confidence are conductor and/or to classify portionsof the image as regions that to a high degree of confidence aresubstrate.

[0048] The second image data includes image data relating to a pluralityof visually distinguishable substrate portions, at least some of whichoverlay conductors, and at least some of which do not overlayconductors. Preferably portions that do not overly conductors areredefined in the second image data as substrate portions that overlayconductors.

[0049] The second image data includes image data that relates to aplurality of visually distinguishable substrate portions, at least someof which are opaque and some of which are non-opaque. Preferably,non-opaque substrate portions are redefined in the second image data asopaque substrate portions.

[0050] The high-sure/low-sure classifier operates on the pseudo image toproduce a high-sure/low-sure image output including at least threeregions as follows: (i) a low-sure region that to a high degree ofconfidence represents only substrate; (ii) a high-sure region that to ahigh degree of confidence represents only conductor located on the topsurface of the electrical circuit; and (iii) a third region which isneither high-sure nor low-sure.

[0051] The high sure/low sure image is employed to selectively modify aninterim image formed from the first image data to produce an enhancedrepresentation of the electrical circuit.

[0052] The first image data is convolved with a mathematical functionapproximating a 2-dimensional Laplacian of a Gaussian function.

[0053] Determining in the first image data approximate locations oftransitions between image regions having distinguishable opticalcharacteristics.

[0054] The enhanced representation is a binary representation of theelectrical circuit.

[0055] The enhanced representation is a representation of contours inthe electrical circuit. Preferably the contours indicate approximatelocations of transitions between regions in the electrical circuit whichexhibit distinguishable optical characteristics.

[0056] The transitions between regions in the electrical circuitexhibiting distinguishable optical characteristics include transitionsbetween substrate and conductors located on a top surface of theelectrical circuit. Preferably, the transitions generally excludetransitions between substrate and other conductors in the electricalcircuit.

[0057] Analyzing the enhanced representation of the electrical circuitto provide an indication of defects in the electrical circuit.

[0058] The first and second image data are acquired with at least twodifferent types of optical detectors which are arranged to view at leasta portion of the electrical circuit as illuminated an illuminator.

[0059] The first and second images of the electrical circuit aregenerally, but not necessarily exactly, spatially coincidental.Preferably each of the first and second images are in a differentspectral range.

[0060] There is thus provided in accordance with another preferredembodiment of the present invention a system for inspecting electricalcircuits comprising: a first image data acquisition assembly obtainingfirst image data relating to part of an electrical circuit: a secondimage data acquisition assembly obtaining second image data generallycorresponding to the same part of the electrical circuit, wherein thesecond image data includes at least some image data that is differentfrom the first image data; a first image data modifier modifying thefirst image data by employing the second image data to produce anenhanced representation of the electrical circuit; and a defectinspector, inspecting the enhanced representation for defects.

[0061] Preferred embodiments of the invention include the preceding andone or more of the following:

[0062] The first image data is in a first spectral range and secondimage data includes at least some image data in a second spectral range.

[0063] A contrast enhancer, enhancing contrast between at least someparts of the second image data which correspond to respective parts ofthe electrical circuit.

[0064] The contrast enhancer enhances contrast in a non-linear manner.

[0065] The contrast enhancer is operative to redefine substrate portionsnot overlaying conductors in the second image data as opaque substrateportions, and thereby generally eliminate any distinction betweensubstrate portions which overlay conductors and substrate portions whichdo not.

[0066] A convolver, convolving the first image data with a function.Preferably the function is an approximation of a Laplacian of a Gaussianfunction, and modifier operates downstream of the convolver.

[0067] A transition locator, determining in the first image dataapproximate locations of transitions between image regions havingdistinguishable optical characteristics and wherein the modifier isoperative to remove undesired transitions.

[0068] The enhanced representation is a binary representation of theelectrical circuit.

[0069] The enhanced representation is a representation of contours inthe electrical circuit. Preferably the contours indicate approximatelocations of transitions between regions in the electrical circuit whichexhibit distinguishable optical characteristics.

[0070] The enhanced representation has a spatial resolution that isgreater than the spatial resolution of either the first or the secondimage data.

[0071] The enhanced representation has a gray scale whose dynamic rangeis reduced as compared with the dynamic range of a gray scale of eitherthe first or the second image data.

[0072] A transition locator, determining in the first image dataapproximate locations of transitions between image regions havingdistinguishable optical characteristics and wherein the modifier isoperative to override at least part of an output of the convolver.

[0073] The first and second data acquisition assemblies include at leastone illuminator and at least one imager, and the imager comprises atleast two different types of optical detectors. Preferably the image isarranged to view at least a portion of the electrical circuitilluminated by the illuminator.

[0074] The imager comprises three types of detectors each of whichoutputs a generally spatially coincidental image of the electricalcircuit in a respective spectral range.

[0075] There is thus provided in accordance with another preferredembodiment of the present invention a system for inspecting electricalcircuits comprising: a first image data acquisition assembly, obtainingfirst image data relating to at least part of an electrical circuit inat least a first spectral range; a second image data acquisitionassembly obtaining second image data relating to at least part of anelectrical circuit in at least a second spectral range; and an enhancedcontrast representation generator providing an enhanced contrastrepresentation of the electrical circuit by non-linearly combining thefirst image data and the second image data.

[0076] Preferred embodiments of the invention include the preceding andone or more of the following:

[0077] The at least part of an electrical circuit includes firstconductors located on the first side of an electrical circuit substrateand second conductors located on the second side of the electricalcircuit substrate, and the enhanced contrast representation includesinformation providing enhanced contrast between representations of thefirst conductors and the substrate.

[0078] The enhanced contrast representation exhibits decreased artifactsresulting from a non-opaque characteristic of the substrate.

[0079] There is thus provided in accordance with another preferredembodiment of the present invention a system for inspecting electricalcircuits formed on different surfaces of a non-opaque substratecomprising: an image data acquisition assembly obtaining image datarelating to at least part of an electrical circuit, and an image dataenhancement assembly, enhancing the image data to provide enhancedinspection output information which decreases artifacts resulting fromthe non-opaque characteristic of the substrate.

[0080] Preferably, the electrical circuits have first conductors on afirst side of the substrate and second conductors on a second side ofthe substrate, and the artifacts include images of the first and secondconductors.

[0081] There is thus provided in accordance with another preferredembodiment of the present invention a system for inspecting electricalcircuits comprising: a first image data acquisition assembly, obtainingfirst image data relating to at least part of an electrical circuit; asecond image data acquisition assembly obtaining second image datarelating to at least part of an electrical circuit; and a pseudo-imagegenerator non-linearly combining the first image data and the secondimage data, the pseudo-image generator being operative to supply apseudo-image of the part of the electrical circuit constructed from thefirst and second image data to a high-sure/low-sure region classifier.

[0082] Preferred embodiments of the invention include the preceding andone or more of the following:

[0083] The second image data includes image data relating to a pluralityof visually distinguishable substrate portions. At least some of thesubstrate portions overlay conductors and the pseudo image generatorredefines substrate portions not overlaying conductors in the secondimage data as substrate portions that overlaying conductors.

[0084] The second image data includes image data relating to a pluralityof visually distinguishable substrate portions. At least some substrateportions are opaque, and the pseudo image generator redefines non-opaquesubstrate portions in the second image data as opaque substrateportions.

[0085] The high-sure/low-sure classifier operates on the pseudo image toproduce a high-sure/low-sure image output which includes at least threeregions as follows: (i) a low-sure region that to a high degree ofconfidence represents only substrate; (ii) a high-sure region that to ahigh degree of confidence represents only conductor located on the topsurface of the electrical circuit; and (iii) a third region which isneither high-sure nor low-sure.

[0086] A representation generator receiving the first image data,wherein the representation generator includes an override circuit incommunication with the high-sure/low-sure classifier and is operative toemploy the high sure/low sure image to selectively modify image data toproduce an enhanced representation of the electrical circuit.

[0087] The representation generator is operative to process the firstimage data.

[0088] A convolver operative to convolve the first image data with amathematical function approximating a dimensional Laplacian of aGaussian function.

[0089] A transition locator operative to determine in the first imagedata approximate locations of transitions between image regions havingdistinguishable optical characteristics.

[0090] The enhanced representation is a binary representation of theelectrical circuit.

[0091] The enhanced representation is a representation of contours inthe electrical circuit. Preferably the contours indicate approximatelocations of transitions between regions in the electrical circuit whichexhibit distinguishable optical characteristics.

[0092] The transitions between regions in the electrical circuit whichexhibit distinguishable optical characteristics include transitionsbetween substrate and conductors located on a top surface of theelectrical circuit. Transitions between substrate and other conductorsin the electrical circuit preferably are not included.

[0093] A defect processor receiving the enhanced representation andoperative to analyze the enhanced representation to provide anindication of defects in the electrical circuit.

[0094] The first and second data acquisition assemblies include at leastone illuminator and at least one imager, wherein the imager includes atleast two different types of optical detectors which are arranged toview a portion of the electrical circuit illuminated by the illuminator.

[0095] The imager comprises three types of detectors, each of which isoperative to output a generally, but not necessarily exactly, spatiallycoincidental image of the electrical circuit in a respective spectralrange.

BRIEF DESCRIPTION OF THE DRAWINGS

[0096] The present invention will be understood and appreciated morefilly from the following detailed description, taken in conjunction withthe drawings in which:

[0097]FIG. 1 is a simplified block diagram of a system for automaticinspection of objects, such as electrical circuits, in accordance with apreferred embodiment of the present invention;

[0098]FIG. 2 is a simplified diagrammatic illustration of methodologyfor generating an enhanced representation of electrical circuits inaccordance with a preferred embodiment of the present invention;

[0099]FIG. 3 is a simplified functional block diagram of apparatus forgenerating an enhanced representation of electrical circuits useful forperforming the methodology of FIG. 2;

[0100]FIG. 4 is a simplified flow chart illustration showing a method ofgenerating an enhanced representation of electrical circuits inaccordance with the methodology of FIG. 2;

[0101]FIGS. 5A, 5B, 5C, 5D, 5E, 5F, 5G, 5H, 5I and 5J are images of aportion of an electrical circuit employed at various stages of themethodology of FIGS. 2-4; and

[0102]FIG. 6 is simplified flow chart illustration of logic used in apreferred embodiment of a method of calculating convolution valuesuseful for generating an enhanced representation of an electricalcircuit.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

[0103] Reference is now made to FIG. 1 which is a simplified blockdiagram of a system 10 for inspecting objects, such as electricalcircuits 12, in accordance with a preferred embodiment of the presentinvention. Electrical circuits 12 that may be inspected by system 10typically include printed circuit boards, ball grid array substrates,bump arrays, flat panel displays, semiconductor devices and othersuitable electrical circuits.

[0104] As seen in FIG. 1, system 10 preferably includes an illuminator14 and an imager 16, typically comprising at least two different typesof optical detectors, indicated by reference numerals 18 and 20. Imager16 views a portion of electrical circuit 12 illuminated by illuminator14. In accordance with a preferred embodiment of the invention, imager16 includes three types of detectors, only two being shown for the sakeof simplicity, such as R, G and B (red, green and blue) detectors eachof which is operative to output a generally spatially coincidental imageof electrical circuit 12 in its respective spectral range. System 10preferably is operative in a scanning arrangement in which electricalcircuit 12 and imager 16 are displaced with respect to each other duringimage acquisition, as known in the art. Alternatively system 10 may beoperative as a staring array system.

[0105] Each of detectors 18 and 20 preferably outputs generallycoincidental optical data corresponding to electrical circuit 12, suchas a digital image thereof. As seen in FIG. 1, a digital image output 22of detector 18, preferably a red detector, is provided to arepresentation generator 24, and a digital image output 26 of detector20 is provided to an enhancer 28. It is noted that the digital output 26may be from a single detector, as seen in FIG. 1, or from multipledetectors as described hereinbelow in greater detail.

[0106] Preferably representation generator 24 outputs an enhanceddigital representation 29 of electrical circuit 12 having a spatialresolution that is greater than the spatial resolution of the digitalimage outputs 22 and 26. Preferably, representation 29 of electricalcircuit 12 exhibits a gray scale whose dynamic range is reduced ascompared with the dynamic range of the gray scale of the digital imageoutput 22. For example, the representation 29 produced by representationgenerator 24 preferably comprises a binary representation of electricalcircuit 12 or a representation of contours in electrical circuit 12.Contours indicate approximate locations of transitions between regionsin electrical circuit 12 exhibiting distinguishable visiblecharacteristics. Such characteristics may include, for example, theintensities of reflection of such regions when exposed to illuminationfrom illuminator 14. It is appreciated that elements forming contoursmay or may not be represented by a data structure that is larger orsmaller than the data structure employed to represent gray scale pixelsin digital output 22.

[0107] In the electrical circuit 12 as seen in FIG. 1, regionsexhibiting distinguishable visible characteristics include a non-opaquesubstrate 30, conductors 32 on a top side of substrate 30, conductors 34on a bottom side of substrate 30 and portions 36 of substrate 30 havingthereon neither of conductors 32 nor 34.

[0108] In a preferred embodiment of the invention, enhancer 28 processesdigital image output 26 to form an enhanced contrast representation ofelectrical circuit 12 that is characterized by synthetically enhancedcontrast between predetermined portions of electrical circuit 12, suchas between conductors 32 and substrate 30. A preferred method ofenhancing contrast includes redefining substrate portions 36 in thedigital image output 26 as opaque substrate portions, such as substrateportions overlaying conductors 34 in an image of electrical circuit 12.Such redefinition thus generally eliminates any distinction betweensubstrate portions which overlay conductors 34 and substrate portions 36which do not. The enhanced contrast representation output of enhancer 28preferably is employed in representation generator 24 to override or toselect portions of digital image output 22, or portions of a result ofan intermediate stage of processing digital image output 22, in thecourse of generation of enhanced representation 29. Alternatively,enhancer 28 may be obviated and the digital image output 26 may besupplied directly as an input to representation generator 24.

[0109] It may thus be appreciated that enhanced representation 29 isgenerated from at least two optical inputs, here digital image inputs 22and the output of enhancer 28, each containing generally spatiallycoincident but different image data, wherein one of the image inputscontrols the other input. Preferably the use of enhanced representation29 in accordance with a preferred embodiment of the present inventionimproves defect detection capabilities in an automated opticalinspection system compared to conventional automated inspection systems.

[0110] As seen in FIG. 1, enhanced representation 29 preferably issupplied to a defect processor 40, which also receives a referencerepresentation 42. Typically reference representation 42 is obtainedfrom the inspection of a non-defective electrical circuit. or is derivedfrom a CAM data file such as may be obtained from a Genesis™ CAM system(not shown), commercially available from Frontline Solutions Ltd. ofYavne, Israel. Preferably processor 40 analyzes enhanced representation29 with reference to one or more design rules governing acceptabledesign parameters of the electrical circuit, the referencerepresentation 42, and a gray scale image output of one of detectors 18and 20 in order to detect defects in electrical circuit 12. Defectprocessor 40 preferably outputs a defect report 44 indicating thepresence of defects that are detected on electrical circuit 12.

[0111] It is appreciated that enhanced representation 29 may be a binaryrepresentation of electrical circuit 12, a contour representationthereof or a representation in another suitable format as required bydefect processor 40. A suitable defect processor 40 including a defectdetection module operative to detect defects in binary representationsof electrical circuits is found in Inspire™ 9060 automated opticalinspection systems and a suitable defect processor including a defectdetection module operative to detect defects in contour representationsof electrical circuits is found in ICP 8060 automated opticalinspections systems, both of which systems are available from OrbotechLtd. of Yavne, Israel.

[0112] Reference is now made to FIG. 2 which is a simplifieddiagrammatic illustration of methodology for generating an enhancedrepresentation 29 (FIG. 1) of an electrical circuit in accordance with apreferred embodiment of the present invention, to FIG. 3 which is asimplified functional block diagram of apparatus generating a suitableenhanced representation 29 useful for performing the methodology of FIG.2, and to FIG. 4 which is a simplified flow chart showing a method ofgenerating the suitable enhanced representation 29 in accordance withthe methodology of FIG. 2.

[0113]FIG. 2 illustrates methodology for inspecting an electricalcircuit 112 employing a color automated optical inspection system 110,such as is found in an Inspire™ 9060 automated optical inspection systemavailable from Orbotech, Ltd. of Israel. System 110 is characterized inthat it contains an imager assembly 116 containing at least one andpreferably three RGB (Red, Green & Blue) detector assemblies 117 (FIG.3), each of which is capable of providing optical data relating toelectrical circuit 112 in multiple channels, such as Red, Green and Bluecolor channels in RGB color space. It is appreciated that each detectorassembly 117 incorporates a plurality of detectors each of whichpreferably provides optical data, such as images, relating to electricalcircuit 112 in a different portion of the electromagnetic spectrum. Itis further appreciated that one or more of the detectors may detectimage data in a non-visible portion of the electromagnetic spectrum,such as x-ray, non-visible UV and IR.

[0114] Initially image data preferably is acquired in parallel by threedetectors 118, 120 and 122 (FIG. 3), preferably providing R, G and Bimage data respectively. Typically electrical circuit 112 appears asseen in FIG. 5A. R, G, and B images typically produced by respective R,G and B detectors 118, 120 and 122, are designated by reference numerals124, 126 and 128 in FIG. 2 and are shown in enlarged form in FIGS. 5B,5C and 5D respectively.

[0115] In the embodiment of FIGS. 2-4, electrical circuit 112 is formedon a non-opaque substrate 130 typically of a type used in printedcircuit board fabrication processes employing double sided treated foils(DSTF). FIG. 5A illustrates, in black and white, the electrical circuit112 as it would be seen by the human eye. It is noted that system 110may or may not employ such an image, in black and white or color, forinspection of the electrical circuit 112.

[0116] Referring to FIG. 5A, it is noted that due to the non-opacity ofsome types of substrate 130, such as substrates used in DSTF processes,there are seen not only the electrical circuit conductors 132 on a topsurface of the circuit but also electrical circuit conductors 134 on oneor more other surfaces of the circuit, and substrate portions 136 thatare not overlaying conductors 134. In order to perform proper inspectionof the conductors 132 on the top surface of the circuit 112, it isnecessary to diminish, or eliminate data relating to substrate portions136 not overlaying conductors 134, which may be viewed by detectors 118,120 and 122, in order to clearly distinguish between data relating toconductors 132 on the top surface of the circuit and substrate portions136. It is appreciated that in other electrical circuits (not shown) itmay be necessary to diminish, or eliminate, data relating to conductors134 in order to clearly distinguish between data relating to conductors132 and conductors 134.

[0117] Turning to FIGS. 5B, 5C and 5D, corresponding to images 124, 126and 128 in FIG. 1, it is seen that the Red, Green and Blue monochromeimages of the electrical circuit each contain image data from conductorson various surfaces of the electrical circuit, but with differingcontrast relationships between the substrate 130, respective conductors132 and 134, and portions 136 not overlaying conductors 134.

[0118] It is a particular feature of the present invention that asuitably weighted combination of the R, G and B image data providesenhanced contrast between image data relating to conductors 132appearing on the top surface of the electrical circuit, other conductors134 and the substrate 130, in the example seen in FIGS. 5A-5D,particularly substrate portions 136 not overlaying conductors 134.Preferably, a non-linear transform of pixel values is performed on theweighted, contrast enhanced, image data to further enhance contrastbetween conductors 132 on a first side of substrate 130 with respect tosubstrate portions overlaying conductors 134 and substrate portions 136not overlaying conductors 134, and thereby to clearly distinguishbetween conductors 132 and all other parts of electrical circuit 112(FIG. 3). Suitable weighting of the R, G and B image data may includenegative weighting factors applied to image data outputs from one ormore of the R, G or B detectors, or selectable weighting factors appliedas a function of various intensity characteristics of pixels in theoutputs of one or more of the R, G, or B detectors. A result fromsuitably weighting R, G and B image data is shown in FIG. 5E.

[0119] As shown in FIG. 3, all three of the image data outputs ofdetectors 118, 120 and 122 preferably are provided to a pseudo-imagegenerator 140. In pseudo-image generator 140 the image data outputs ofdetectors 118, 120 and 122 are combined with a desired relativeweighting, and the values of selected pixels are transformed, to producea synthesized image, an example of which is seen in FIG. 5E. Thissynthesized image is termed a “pseudo-image” and is designated in FIG. 2by reference numeral 142.

[0120] In accordance with a preferred embodiment of the invention, inthe operation of the pseudo-image generator 140, weighting coefficientsare heuristically assigned on the basis of empirical analysis of one ormore typical electrical circuits to be inspected in order to suitablyenhance contrast between various portions of interest in an electricalcircuit being inspected. For example, if it is desired to distinguishbetween conductors 132 and substrate portions 136 not overlayingconductors 134, so that in pseudo-image 142 conductors 132 are contrastenhanced and readily distinguishable from all other portions ofelectrical circuit 112, a color image of an electrical circuit 112 to berested may be analyzed in order to suitably characterize substrateportions 136 and to assign contrast enhancing weighting coefficients.Preferably threshold values are also determined to so that the values ofpixels in the contrast enhanced image of electrical circuit 112 thatexceed (or that do not exceed) the threshold are transformed such thatin the pseudo-image 142, substrate portions 136 artificially appear asif they are opaque, such as substrate portions that overlay conductors134. It is appreciated that such a non-linear transform further enhancesthe contrast of, and distinguishes, conductors 132 with respect to otherparts of electrical circuit 112.

[0121] The empirical analysis employed for determining weighting mayindicate, for example as seen in FIGS. 5B, 5C and 5D, that:

[0122] a) conductors 132 and substrate portions 136 not overlayingconductors 134 both exhibit an intensity level that is greater than thatof substrate portions overlaying conductors 134 in each of the R, G andB outputs 124, 126 and 128 respectively;

[0123] b) substrate portions 136 generally exhibit a lower intensitylevel than the intensity level of conductors 132 in the R output 124;

[0124] c) substrate portions 136 generally exhibit a higher intensitylevel than that of conductors 132 in the G output 126; and

[0125] d) substrate portions 136 generally exhibit the same intensitylevel as that of conductors 132 in the B output 128.

[0126] If it is desired to de-emphasize substrate portions 136 ascompared to substrate portions overlaying conductors 134, so as toenhance the contrast between conductors 132 on the one hand and allportions of substrate 130 on the other hand, it can be seen that the Goutput should be given a relatively low weighting, or even a negativeweighting as compared to the R and B outputs and that the B outputshould be given a relatively low weighting as compared to the R output.

[0127] In accordance with a preferred embodiment of the invention, R, Gand B outputs from detectors 118, 120 and 122 respectively are assignedsuitable weightings based upon the above-described analysis, and theircomposite value is evaluated with reference to a threshold as follows:

(a×Red)+(b×Green)+(c×Blue)>d

[0128] where “a” is a weighting coefficient assigned to pixels in theRed output, “b” is a weighting coefficient assigned to pixels in theGreen output, “c” is a weighting coefficient assigned to pixels in theBlue output and “d” is a threshold value that the composite of theweighted values for R, G and B must exceed in order for a pixel to beconsidered not to represent substrate. Thus when suitable weightings areapplied to a digital image, with reference to FIGS. 5B-5D, the weightedvalues of pixels representing substrate portions 136 not overlayingconductors 134 do not exceed the threshold and therefore their valuesare transformed, preferably non-linearly, in the pseudo-image so thatthey correspond to pixels representing an opaque substrate, such assubstrate portions overlaying conductors 134, while the weighted valuesof pixels representing substrate portions overlaying conductors 134 andconductors 132 exceed the threshold and therefore are not transformed.

[0129] Another suitable method of enhancing contrast between conductors132 and other parts of the electrical circuit 112 involves comparingweighted image input values obtained from different image outputs anddeciding how to represent pixels based on the results of suchcomparison. For example, if the G value of a pixel in a digital image ofelectrical circuit 112 exceeds a suitably weighted value for Red (a×Red)of that pixel, then the pixel value of that pixel in the pseudo-image142 is transformed, preferably non-linearly, to provide contrast withconductors 132. Preferably, such a transformed pixel receives a “0”value representing an opaque substrate, or a value similar to a pixelrepresenting substrate overlaying conductor 134, although in actualityit represents, for example, a substrate portion 136. If the value of thepixel does not exceed the weighted value for Red, its value is nottransformed. A suitable value for the coefficient “a” typically isselected heuristically and, depending on color populations present in anelectrical circuit under inspection, may be in the range, for example,1.2-1.5.

[0130] Another suitable contrast enhancing and conductor distinguishingmethod may employ other logic applied to respective pixel values in adigital image of electrical circuit 112. For example, the value ofpixels meeting either of the following conditions may be transformed inthe psuedo-image 142 to suitably contrast pixels representing conductors132, such as with a “0” value, a value that corresponds to an opaquesubstrate or another value that corresponds to substrate overlayingconductors 134:

[0131] a) Green is greater than (a×Red) AND Green is less than “e” ANDRed is less than “e*”; OR

[0132] b) Green is greater than (f×Red) AND Green is greater than “e”

[0133] where “a” is a weighting coefficient assigned to pixels in theRed output; “e” and “e*” are threshold values for a pixel intensityvalue of the Red and Green image outputs respectively; and “f” isanother weighting coefficient assigned to pixels in the Red imageoutput. Each of “a”, “e” “e*” and “f” are selected preferably based on aheuristic analysis of the electrical circuit so as to produce a desiredcontrast enhancement. A suitable value for the coefficient “a” may be inthe range of, for example, 0.7-1.2. A suitable value for the “f”coefficient may be in the range of, for example, 0.9-1.5. Suitablevalues for “e” and “e*” respectively maybe in the range of, for example,175-225 on a scale of 256 for possible intensity values.

[0134] Weighted pixels meeting either of the above conditions may be,for example, pixels representing regions of substrate 136 of anon-opaque substrate 130. The values of such pixels are transformed inthe psuedo-mage 142 so as to contrast pixels representing conductors 132by assigning a synthetic value such as “0”, a value representative of anopaque substrate, or a value representative of a substrate portion thatdoes overlay conductors 134. Pixels meeting neither of the conditions,for example pixels representing conductors 132 on the top side ofelectrical circuit 112, or pixels representing substrate portionsoverlaying conductors 134, preferably are not transformed in thepsuedo-image 142 and retain, for example, their actual R image outputvalue.

[0135] It is appreciated that the above analyses may be performed on aregional, pixel by pixel or other suitable basis, and may be performedin software and/or hardware and/or by using suitable look-up tables, asappropriate.

[0136] Thus, it is appreciated that generation of a contrast enhancedimage, such as the pseudo-image of FIG. 5E, preferably results from thefollowing steps:

[0137] obtaining first image data relating to at least part of anelectrical circuit in at least a first spectral range;

[0138] obtaining second image data relating to at least part of anelectrical circuit in at least a second spectral range;

[0139] optionally, obtaining third image data relating to at least partof an electrical circuit in at least a third spectral range, and

[0140] combining information from the first image data, the second imagedata, and optionally the third image data, optionally with predeterminedweightings, and transforming the values of at least some pixels in anon-linear manner, to provide combined image data containing enhancedcontrast inspection output information.

[0141] Returning now additionally to FIG. 3, it is seen that the outputof pseudo-image generator 140, typically pseudo-image 142 (seen ingreater detail in FIG. 5E), is provided to a high-sure/low-sure regionclassifier 144. Pseudo-image generator 140 and high-sure/low-sureclassifier typically are included in enhancer 28 (FIG. 1) andhigh-sure/low-sure classifier 144 operates on the pseudo image 142 toproduce a high-sure/low-sure image output 146, which is seen in greaterdetail in FIG. 5F. Referring to FIG. 5F, it is seen that thehigh-sure/low-sure image output 146 includes three regions: (i) alow-sure region 148, which is a region that to a high degree ofconfidence represents only substrate 130 (FIG. 3), including bothsubstrate portions 136 and substrate portions that overlay conductors134, (ii) a high-sure region 150, which is a region that to a highdegree of confidence represents only conductors 132 located on the topsurface of an electrical circuit 112, and (iii) a third region 152 whichis neither high-sure nor low-sure.

[0142] In accordance with a preferred embodiment of the invention,classification of pixels in the pseudo-image 142 as belonging to one ofregions 148, 150 and 152 is performed in classifier 144 by thresholding.Preferably a suitable threshold value is chosen which is determinedheuristically, preferably as a function of R intensities characteristicof a particular electrical circuit to be inspected. In accordance with apreferred embodiment of the invention, in order for a pixel in an imageto be deemed high-sure, namely belonging to region 150, the value of thepixel and each of its eight immediately surrounding pixel neighbors mustexceed the threshold value. If the value of the pixel and each of itseight immediately surrounding pixel neighbors is less than the thresholdvalue, then the pixel is deemed low-sure, namely belonging to region148. AU other pixels are deemed to belong to region 152.

[0143] It is noted that in accordance with a preferred embodiment of theinvention, a single threshold value may be used to distinguish betweenhigh-sure pixels 150 and low-sure pixels 148, or separate thresholdvalues may applied to the determination of high-sure pixels 150 orlow-sure pixels 148 respectively. If separate thresholds are used todetermine if a pixel is high-sure or low-sure, and a pixel meets thethreshold test to be deemed a high sure pixel 150, and additionallymeets the threshold test to be deemed a low sure pixel 148, thenpreferably the pixel is deemed to be a low sure pixel 148.

[0144] The functions of pseudo image generator 140 and high-sure/lowsure region classifier 144 preferably are performed in enhancer 28 (FIG.1), and the high-sure/low sure image output 146, which is an enhancedcontrast image, is provided to an override circuit 160 which preferablyis part of representation generator 24.

[0145] As seen in FIG. 3 override circuit 160 preferably receives afirst input 162 from a convolver 164 which receives monochrome imagedata from imager assembly 117, preferably but not necessarily an R imageoutput 124. Convolver 164 carries out a two-dimensional convolution onthe monochrome image data, which preferably is operative tomathematically modify the values of pixels in the image output 124 andto produce a convolved image output, an example of which is shown inFIG. 5G, which constitutes first input 162.

[0146] In accordance with a preferred embodiment of the invention, inconvolver 164 a convolution value is calculated for each pixel in the Rimage output 124 by convolving the pixels in the R image output 124,each of which has a value which is a function of the intensity ofreflected light at that pixel, with a 2-dimensional Laplacian of aGaussian function, or with an approximation thereof. Preferably theconvolution of pixels in R image output 124 with an approximation of theLaplacian is performed using a Difference of Gaussians (DOG)methodology, substantially as described in U.S. Pat. No. 5,774,572, thedisclosure of which is hereby incorporated herein.

[0147] In accordance with the DOG methodology, the convolution with anapproximation of a 2-dimensional Laplacian of a Gaussian functionpreferably is performed by first calculating a value for the convolutionof a 5×5 array of pixels with a 2-dimensional approximation of aGaussian function. Subsequently, the value of the central pixel of the5×5 array is subtracted from the convolution result calculated for theconvolution of the entire 5×5 array with the 2-dimensional approximationof a Gaussian function. It is appreciated that the value of the centralpixel corresponds to a convolution with a 1×1 Gaussian function. Theresult of the subtraction is the convolution value for the central pixelof the 5×5 array of pixels. In a preferred embodiment of the invention,the value for the convolution of the 5×5 array with a Gaussian functionis calculated using a repeated boxcar function applied two dimensionallyon successive pixel pairs, and summing the result obtained for the 5×5array of pixels.

[0148] It is appreciated that the DOG methodology for convolving a pixelarray with an approximation of a Laplacian of a Gaussian function may beperformed by calculating convolutions with a Gaussian function onvarious sizes of pixel arrays. For example a satisfactory convolutionmay be obtained by convolving a 9×9 pixel array with an approximation ofa Gaussian function and subtracting the result of convolving a 3×3 pixelarray with an approximation of a Gaussian function. Other sizes of pixelarrays may also be used.

[0149] Alternatively, the following kernel, providing values suitablefor convolving a 5×5 array of pixels with a two dimensionalapproximation of a Laplacian of a Gaussian function, is particularlyappropriate for software implementations: 0.0039 0.0156 0.0234 0.01560.0039 0.0156 0.0625 0.0938 0.0625 0.0156 0.0234 0.0938 −0.8594   0.09380.0234 0.0156 0.0625 0.0938 0.0625 0.0156 0.0039 0.0156 0.0234 0.01560.0039

[0150] A convolution value for a pixel corresponding to the central cellof the kernel is the sum of reflective intensity values for neighboringpixels, corresponding to cells in the kernel, wherein each intensityvalue is multiplied by a value taken from a corresponding cell in thekernel.

[0151] Alternatively, the convolution value may be performed in multiplesteps as follows:

[0152] An initial convolution of the image is performed with thefollowing kernel: 0 0 0.0625 0 0 0 0 0.25  0 0 0 0 0.375  0 0 0 0 0.25 0 0 0 0 0.0625 0 0

[0153] The output of the above convolution is further convolved with thefollowing kernel: 0 0 0 0 0 0 0 0 0 0 0.0625 0.25 0.375 0.25 0.0625 0 00 0 0 0 0 0 0 0

[0154] The output of the second convolution thereafter is summed withthe following kernel: 0 0 0 0 0 0 0 0 0 0 0 0 −1   0 0 0 0 0 0 0 0 0 0 00

[0155] The result of the summation is a convolved image, whichcorresponds to image output 162.

[0156] Reference is made to FIG. 5G which shows the convolved imageoutput which defines image output 162, which is the result of convolvingR image output 124 as hereinabove described. Preferably. the convolvedimage output 162 has substantially the same resolution as R image output124, however the gray level values of the pixels in the convolved imageoutput 162 are redefined as a result of the convolution. As seen in FIG.5G, convolution of the R image output 124 provides a representation ofelectrical circuit 112 (FIG. 3).

[0157] In the convolved image output 162 shown in FIG. 5G, regions thatcorrespond to locations in R image output 124 (FIG. 5B) which exhibitgenerally uniform intensity of reflected light, for example regions ofsubstrate 166 and regions of a conductor 168 in the convolved imageoutput 162, have a convolution value that is 0 or near 0, and appear inintermediate shades of gray. Regions in convolved image ouput 162 thatcorrespond to locations in R image output 124 (FIG. 5B) which exhibit arelatively strong spatial transition in reflective intensity, such aslocations near the edges of conductors 132, have relatively largepositive convolution values or relatively large negative convolutionvalues, as compared with the convolution values calculated for regions166 and 168, and appear as relatively light and relatively dark regions,designated respectively by reference numerals 170 and 172.

[0158] It is noted that typically within regions 166 and 168 there maybe a variation in the gray scale values assigned to individual pixels.These values may be zero or small numbers above or below zero.

[0159] As seen in FIG. 5G, the substrate side of an edge of a conductortypically is assigned a relatively large positive value and appears as alight region 170, and the conductor side of an edge typically isassigned a relatively large negative value and appears as a dark region172.

[0160] In order for the functionality of the override circuit 160 to beunderstood and appreciated, it is believed that a brief overview of thegeneration of enhanced representation 29 (FIG. 3) from the convolvedimage output 162 is in order.

[0161] In accordance with a preferred embodiment of the invention,enhanced representation 29 is produced by calculating sub-pixellocations of corresponding zero crossings between pixels havingoppositely signed values, preferably based on a linear interpolationbetween the oppositely signed values. Each zero crossing betweenoppositely signed pixels represents an approximate location of atransition between regions exhibiting optically distinguishablecharacteristics. However, as mentioned hereinabove, the convolution of Rimage output 124 typically results in near zero positive and near zeronegative values for regions in convolved image output 162, such asregions 166 and 168, corresponding to locations in the R image output124 which exhibit nearly uniform intensity of light reflectiontherefrom. Hence, override circuit 160 normally is required.

[0162] As seen in FIG. 3, convolved image output 162 andhigh-sure/low-sure image output 146 are supplied to override circuit160, which modifies the convolved image output 162 so that pixels inregions 166 and 168 have uniformly signed (positive or negative) values.Override circuit 160 preferably modifies convolved image output 162using a controlling input containing at least some image informationthat is different from image information contained in convolved imageoutput 162, preferably at least partially received from a detector otherthan the detector providing an output employed by convolver 164. In apreferred embodiment of the invention, the high-sure/low sure imageoutput 146 supplied by high-sure/low-sure region classifier 144 providesthe controlling input, preferably in the manner described hereinbelow:

[0163] Convolved image 162 and high-sure/low sure image output 146preferably are analyzed pixel by pixel in override circuit 160. It isappreciated that regions 148 and 150 in high-sure/low-sure image output146 (FIG. 5F), representative of substrate 130 and conductor 132 (FIG.3) respectively, preferably each have a characteristic positive ornegative sign. Pixels in convolved i m a g e 162 that correspond topixels located in either of low-sure regions or high-sure regions ofhigh-sure/low sure image output 146, such as respective regions 148 and150 (FIG. 5F), are modified so that they have the same positive ornegative sign as the corresponding pixel in the high-sure/low sure imageoutput 146. Thus, for example, if a pixel in region 166 of convolvedimage 162 has a negative value which is characteristic of conductor, butaccording to high-sure/low-sure image output 146 the pixel is a low-surepixel which should have a positive value characteristic of substrate,then the pixel value for the pixel in convolved image 162 is modified byassigning to it a predetermined positive value that is representative ofsubstrate.

[0164] The result of operation of override circuit 160 typically is anoverride map 174 in which the values of some pixels, corresponding topixels in regions 148 and 150 of high-sure/low sure image output 146,have been modified so that they receive a correct sign which avoidszero-crossings among those pixels. Preferably override map 174 has thesame resolution as convolved image 162.

[0165] Reference is made to FIG. 5H which shows a modified override map175 which has been modified for clarity of illustration. Modifiedoverride map 175 is a rendition of override map 174 in which, forclarity of illustration, those pixels corresponding to pixels in regions166 and 168 of FIG. 5G have been assigned binary values according totheir respective positive and negative signs. Thus, regions 176 in FIG.5H, corresponding to regions 166 in FIG. 5G and having positive signs,appear white and regions 178 in FIG. 5H, corresponding to regions 168 inFIG. 5G and having negative signs, appear black.

[0166] It is noted that modified override map 175 typically is not usedin system 110 and that the large scale of the overrides illustratedtherein does not normally take place in reality. Modified override map175 is provided here in order to illustrate the override mechanism. Anaccurate illustration of override map 174 would not be illustrative ofthe override mechanism because the modifications made by overridecircuit 160 to convolved image 162 to generate override map 174typically are so subtle that override map 174 is nearly visuallyindistinguishable from convolved image 162. Moreover, it is appreciatedthat the grayscale values of pixels in override map 174 are employed toapproximate the locations of transitions between oppositely signedpixels.

[0167] As further seen in FIG. 3, override map 174 preferably isprovided to a zero crossing calculator 179 which operates on overridemap 174 to calculate zero-crossings between adjacent pixels thereinhaving oppositely signed values. It is appreciated that followingoperation of override circuit 160, adjacent pixels having oppositelysigned values typically exist only among pixels in override map 174which correspond to pixels located in third region 152 (FIG. 5F), namelypixels which are neither high-sure 150 nor low-sure 148. Generally,pixels corresponding to pixels located in third regions 152 exhibitvarious degrees of gray, scale. It is further appreciated that byemploying high-sure/low-sure image output 146 as a controlling input tooverride circuit 160, zero-crossings preferably are calculated only fortransitions between conductors 132 and substrate 130, and zero-crossingsare not calculated for transitions between conductors 134 and substrate136.

[0168] In accordance with a preferred embodiment of the invention,transitions between conductors 132 and substrate 130 are deemed to existsomewhere along imaginary lines connecting the center points of a pairof adjacent side-by-side pixels in override map 174 which haveoppositely signed values. The determination of the location of a zerocrossing, which approximates the location of a transition betweenconductor 132 and substrate 130, preferably is made by linearinterpolation between the respective values (appearing in for example inFIG. 5G as various shades of gray) of oppositely signed convolutionvalues of adjacent pixels, as described more fully in U.S. Pat. No.5,774,572, the disclosure of which is hereby incorporated by reference.

[0169] Referring additionally to FIG. 2, the result of calculation ofzero crossings in override map 174 (shown in FIG. 2 as the modifiedoverride map 175) is provided to an enhanced representation producer 180which produces enhanced representation 29. Preferably the zero-crossingsare connected to provide a contour representation 182 (seen in greaterdetail in FIG. 5I) of the outline of conductors 132 in electricalcircuit 112. It is appreciated that the resulting outline connecting thezero-crossings is drawn to sub-pixel accuracy. Alternatively oradditionally, a binary representation 184 (seen in greater detail inFIG. 5J) of conductors 132 and substrate 130 in electrical circuit 112is produced by utilizing the locations of the zero crossings. The binaryrepresentation preferably has a resolution which is spatially enhancedrelative to image outputs 124, 126 and 128.

[0170] Preferably pixels in a binary representation, such as binaryrepresentation 184, are grouped into multi-bit computational pixelshaving a data structure that indicates the presence or absence of atransition between dark and light regions located inside thecomputational pixel. Additionally, the data structure preferablyindicates a sub-computational pixel location of such a transition forcomputational pixels in which such a transition occurs. A suitablemethod of representing binary images with multi-bit computational pixelsis described in published PCT patent application WO 0019372,incorporated herein by reference.

[0171] Reference is now made to FIG. 6 which is a simplified flow chartillustration showing preferred logic employed to calculate pixel valuesin override map 174 (FIG. 3), which map is used to provide enhancedrepresentation 29 of an electrical circuit 112 being inspected. Red,Green and Blue image outputs 124, 126 and 128 are acquired fromrespective detectors 118, 120 and 122 (FIG. 3). Red image output 124 isthen used to calculate a first approximation to the Laplacian of aGaussian (LOG) preferably by using a repeated boxcar function (200).Preferably the LOG is obtained using a difference of Gaussians (DOG)methodology by convolving 5×5 neighboring pixels with a 2-dimensionalapproximation of a Gaussian function obtained from the boxcar function,and then subtracting the value of the central pixel from the 5×5convolution result substantially as described hereinabove.

[0172] In parallel, in the method seen in FIG. 6 a look-up table isapplied to Red, Green and Blue outputs 124, 126 and 128 in order toclassify substrate 130 (FIG. 3) based on color so as to enhance contrastbetween substrate 130 and conductors 132, while removing image artifactsassociated with non-opaque portions of substrate 130. This step isdesignated by reference numeral 210. The contrast enhanced imageresulting from step 210, for example pseudo image 142 (FIG. 5E) is thenused to produce a high-sure image and a low-sure image in steps 220 and230 respectively. The resulting outputs, designated respectively as 222and 232 define high-sure/low-sure image output 146 (FIG. 5F).

[0173] In producing the low sure image 232 in step 230, each pixel inthe contrast enhanced image provided in step 210 is thresholded andpreferably is assigned a value of 1 if the pixel itself and all of its 8immediate neighboring pixels appear to be substrate. In producing thehigh sure image 222 in step 220, each pixel in the contrast enhancedimage is thresholded and preferably is assigned a value of 1 if thepixel itself and all of its 8 immediate neighbors appear to beconductor.

[0174] Although the high sure and low sure images in the illustratedmethodology are shown as being produced on the basis of an enhancedcontrast image, it is appreciated that the production of high sure andlow sure images may employ any suitable controlling input includinginformation which is not present in the image input used to calculatethe LOGs in step 200, provided that artifacts relating to undesiredportions of the electrical circuit being inspected are absent from thecontrolling input.

[0175] A logic sequence is now applied in order to determine whether apixel retains its LOG value as calculated in step 200, or rather isassigned a LOG value based on evaluation with respect to thehigh-sure/low sure image produced in steps 220 and 230 respectively.

[0176] First, taking the output 232 of step 230, a pixel is analyzed ina step 240 in order to determine whether it is a low sure pixel (LS). Ifthe pixel is a low sure pixel its LOG is evaluated in a step 250 inorder to determine whether the LOG has a positive or negative value. Ifthe LOG of a low sure pixel has a positive value, its LOG value is notchanged, however if the LOG of a low sure pixel has a negative value,its initial LOG value is assigned a positive value of 1, which isrepresentative of substrate.

[0177] Referring now to output 222 of step 220, and to pixels in theoutput 232 which are not low sure pixels, each pixel is analyzed in step260 to determine whether it is a high sure pixel (HS). If the pixel isnot a high sure pixel, its LOG value is left unchanged. However if thepixel is a high sure pixel, its LOG is value is evaluated in step 270 todetermine whether its value is positive or negative. If the LOG value ofa high sure pixel is negative, then it is representative of conductorand its LOG value is left unchanged. However if the LOG value of a highsure pixel is positive, its LOG value is assigned a negative value of−1, which is representative of conductor, such as conductor 132 on thetop surface of substrate 130 (FIG. 3).

[0178] The LOGs resulting from the logic described with reference toFIG. 6 are collected into override map 174 (FIG. 3) which is provided tozero crossing generator 179 (FIG. 3).

[0179] It is appreciated by persons skilled in the art that the presentinvention is not limited to what has been particularly shown anddescribed hereinabove. Rather, the scope of the present inventionincludes various features described hereinabove as well as modificationsand additions thereto which would occur to a person of skill in the artupon reading the foregoing description and which are not in the priorart. By way of non-limiting example, although the invention has beendescribed in the context of a preferred methodology in which convolvedimage 162 is modified with high-sure/low-sure image output 146 prior tocalculating zero crossings, it is to be appreciated by persons skilledin the art that zero crossing may be first calculated directly onconvolved image 162, and that erroneous zero crossings may be filteredusing a controlling input, such as high-sure/low-sure image output 146.

1. A method of inspecting electrical circuits comprising: obtainingfirst image data relating to at least a part of an electrical circuit;obtaining second image data generally corresponding to said part of anelectrical circuit, said second image data including at least some imagedata that is different from said first image data; modifying said firstimage data by employing said second image data thereby to produce anenhanced representation of the electrical circuit; and inspecting theenhanced representation for defects.
 2. A method of inspectingelectrical circuits according to claim 1 and wherein said first imagedata is in a first spectral range and second image data includes atleast some image data in a second spectral range.
 3. A method ofinspecting electrical circuits according to claim 1 and also comprising:enhancing contrast between at least some parts of said second image datarepresenting corresponding parts of the electrical circuit.
 4. A methodof inspecting electrical circuits according to claim 3 and wherein saidenhancing contrast is non-linear.
 5. A method of inspecting electricalcircuits according to claim 3 and wherein said enhancing contrastincludes redefining substrate portions not overlaying conductors in saidsecond image data as opaque substrate portions, thus generallyeliminating any distinction between substrate portions which overlayconductors and substrate portions which do not.
 6. A method ofinspecting electrical circuits according to claim 2 and also comprising:enhancing contrast between at least some parts of said second image datarepresenting corresponding parts of the electrical circuit.
 7. A methodof inspecting electrical circuits according to claim 6 and wherein saidenhancing contrast is non-linear.
 8. A method of inspecting electricalcircuits according to claim 1 and also comprising: convolving said firstimage data with a function.
 9. A method of inspecting electricalcircuits according to claim 8 and wherein said function is anapproximation of a Laplacian of a Gaussian function.
 10. A method ofinspecting electrical circuits according to claim 8 and wherein saidmodifying is carried out following said convolving.
 11. A method ofinspecting electrical circuits according to claim 6 and also comprising:convolving said first image data with a function.
 12. A method ofinspecting electrical circuits according to claim 11 and wherein saidfunction is an approximation of a Laplacian of a Gaussian function. 13.A method of inspecting electrical circuits according to claim 11 andwherein said modifying is carried out following said convolving.
 14. Amethod of inspecting electrical circuits according to claim 1 and alsocomprising: determining in said first image data approximate locationsof transitions between image regions having distinguishable opticalcharacteristics; and wherein said modifying comprises removing undesiredones of said transitions.
 15. A method of inspecting electrical circuitsaccording to claim 1 and wherein said enhanced representation is abinary representation of said electrical circuit.
 16. A method ofinspecting electrical circuits according to claim 1 and wherein saidenhanced representation is a representation of contours in saidelectrical circuit, which indicate approximate locations of transitionsbetween regions in said electrical circuit exhibiting distinguishableoptical characteristics.
 17. A method of inspecting electrical circuitsaccording to claim 1 and wherein said enhanced representation has aspatial resolution that is greater than the spatial resolution of saidfirst and second image data.
 18. A method of inspecting electricalcircuits according to claim 17 and wherein said enhanced representationhas a gray scale whose dynamic range is reduced as compared with thedynamic range of a gray scale of said first and second image data.
 19. Amethod of inspecting electrical circuits according to claim 8 and alsocomprising: determining in said first image data approximate locationsof transitions between image regions having distinguishable opticalcharacteristics; and wherein said modifying includes overriding at leastpart of said convolved first image data.
 20. A method of inspectingelectrical circuits according to claim 8 and wherein said enhancedrepresentation is a binary representation of said electrical circuit.21. A method of inspecting electrical circuits according to claim 8 andwherein said enhanced representation is a representation of contours insaid electrical circuit, which indicate approximate locations oftransitions between regions in said electrical circuit exhibitingdistinguishable optical characteristics.
 22. A method of inspectingelectrical circuits according to claim 8 and wherein said enhancedrepresentation has a spatial resolution that is greater than the spatialresolution of said first and second image data.
 23. A method ofinspecting electrical circuits according to claim 22 and wherein saidenhanced representation has a gray scale whose dynamic range is reducedas compared with the dynamic range of a gray scale of said first andsecond image data.
 24. A method of inspecting electrical circuitsaccording to claim 1 and wherein said first and second images areacquired with at least one imager comprising at least two differenttypes of optical detectors arranged to view at least a portion of saidelectrical circuit illuminated by at least one illuminator.
 25. A methodof inspecting electrical circuits according to claim 24 and wherein saidfirst and second images are generally spatially coincidental, and eachof said first and second images of is in a different spectral range. 26.A method of inspecting electrical circuits comprising: obtaining firstimage data relating to at least part of an electrical circuit in atleast a first spectral range; obtaining second image data relating to atleast part of an electrical circuit in at least a second spectral range;and providing an enhanced contrast representation of the electricalcircuit by non-linearly combining said first image data and said secondimage data.
 27. A method of inspecting electrical circuits according toclaim 26 and wherein said at least part of an electrical circuitincludes first conductors located on a first side of an electricalcircuit substrate and second conductors located on a second side of anelectrical circuit substrate and wherein said enhanced contrastrepresentation includes information providing enhanced contrast betweenrepresentations of said first conductors and of said electrical circuitsubstrate.
 28. A method of inspecting electrical circuits according toclaim 26 and wherein said enhanced contrast representation exhibitsdecreased artifacts resulting from a non-opaque characteristic of asubstrate.
 29. A method of inspecting electrical circuits formed ondifferent surfaces of a non-opaque substrate comprising: obtaining imagedata relating to at least part of an electrical circuit and enhancingsaid image data to provide enhanced inspection output information whichdecreases artifacts resulting from the non-opaque characteristic of thesubstrate.
 30. A method of inspecting electrical circuits according toclaim 29 wherein the electrical circuits comprise first conductors on afirst side of the substrate and second conductors on a second side ofthe substrate, and the artifacts include at least part of an image froma substrate portion not having deposited thereon one of said first andsecond conductors.
 31. A method of inspecting electrical circuitscomprising: obtaining first image data relating to at least part of anelectrical circuit; obtaining second image data relating to at leastpart of an electrical circuit; and non-linearly combining said firstimage data and said second image data to form a pseudo image, andsupplying said pseudo-image to a high-sure/low-sure region classifier.32. A method of inspecting electrical circuits according to claim 31 andwherein said second image data includes image data relating to aplurality of visually distinguishable substrate portions, at least somesubstrate portions overlaying conductors, and substrate portions notoverlaying conductors are redefined in said second image data assubstrate portions overlaying conductors.
 33. A method of inspectingelectrical circuits according to claim 31 and wherein said second imagedata includes image data relating to a plurality of visuallydistinguishable substrate portions, at least some substrate portionsoverlaying conductors, and non-opaque substrate portions are redefinedin said second image data as opaque substrate portions.
 34. A method ofinspecting electrical circuits according to claim 31 and wherein saidhigh-sure/low-sure classifier operates on the pseudo image to produce ahigh-sure/low-sure image output including at least three regions: (i) alow-sure region that to a high degree of confidence represents onlysubstrate; (ii) a high-sure region that to a high degree of confidencerepresents only conductor located on the top surface of said electricalcircuit; and (iii) a third region which is neither high-sure norlow-sure.
 35. A method of inspecting electrical circuits according toclaim 34 further comprising: receiving said first image data andemploying said high sure/low sure image to selectively modify an interimimage formed from said first image data to produce an enhancedrepresentation of said electrical circuit.
 36. A method of inspectingelectrical circuits according to claim 35 further comprising: convolvingsaid first image data with a mathematical function approximating a2-dimensional Laplacian of a Gaussian function.
 37. A method ofinspecting electrical circuits according to claim 36 further comprising:determining in said first image data approximate locations oftransitions between image regions having distinguishable opticalcharacteristics.
 38. A method of inspecting electrical circuitsaccording to claim 35 and wherein said enhanced representation is abinary representation of said electrical circuit.
 39. A method ofinspecting electrical circuits according to claim 35 and wherein saidenhanced representation is a representation of contours in saidelectrical circuit, which indicate approximate locations of transitionsbetween regions in said electrical circuit exhibiting distinguishableoptical characteristics.
 40. A method of inspecting electrical circuitsaccording to claim 39 and wherein said transitions between regions insaid electrical circuit exhibiting distinguishable opticalcharacteristics include transitions between substrate and conductorslocated on a top surface of said electrical circuit, and generallyexclude transitions between substrate and other conductors in saidelectrical circuit.
 41. A method of inspecting electrical circuitsaccording to claim 35 further comprising: analyzing said enhancedrepresentation to provide an indication of defects in said electricalcircuit.
 42. A method of inspecting electrical circuits according toclaim 31 and wherein said first and second image data are acquired withat least two different types of optical detectors arranged to view atleast a portion of said electrical circuit illuminated by at least oneilluminator.
 43. A method of inspecting electrical circuits according toclaim 42 and wherein said first and second images of said electricalcircuit are generally spatially coincidental, and are each in adifferent spectral range.
 44. A system for inspecting electricalcircuits comprising: a first image data acquisition assembly obtainingfirst image data relating to at least a part of an electrical circuit; asecond image data acquisition assembly obtaining second image datagenerally corresponding to said part of said electrical circuit, saidsecond image data including at least some image data that is differentfrom said first image data; a first image data modifier modifying saidfirst image data by employing said second image data thereby to producean enhanced representation of the electrical circuit; and a defectinspector, inspecting the enhanced representation for defects.
 45. Asystem for inspecting electrical circuits according to claim 44 andwherein said first image data is in a first spectral range and secondimage data includes at least some image data in a second spectral range.46. A system for inspecting electrical circuits according to claim 44and also comprising: a contrast enhancer, enhancing contrast between atleast some parts of said second image data representing correspondingparts of the electrical circuit.
 47. A system for inspecting electricalcircuits according to claim 46 and wherein said contrast enhancerenhances contrast in a non-linear manner.
 48. A system for inspectingelectrical circuits according to claim 46 and wherein said contrastenhancer is operative to redefine substrate portions not overlayingconductors in said second image data as opaque substrate portions, thusgenerally eliminating any distinction between substrate portions whichoverlay conductors and substrate portions which do not.
 49. A system forinspecting electrical circuits according to claim 45 and alsocomprising: a contrast enhancer, enhancing contrast between at leastsome parts of said second image data representing corresponding parts ofthe electrical circuit.
 50. A system for inspecting electrical circuitsaccording to claim 49 and wherein said contrast enhancer enhancescontrast in a non-linear manner.
 51. A system for inspecting electricalcircuits according to claim 44 and also comprising: a convolver,convolving said first image data with a function.
 52. A system forinspecting electrical circuits according to claim 51 and wherein saidfunction is an approximation of a Laplacian of a Gaussian function. 53.A system for inspecting electrical circuits according to claim 51 andwherein said modifier operates downstream of said convolver.
 54. Asystem for inspecting electrical circuits according to claim 49 and alsocomprising: a convolver, convolving said first image data with afunction.
 55. A system for inspecting electrical circuits according toclaim 54 and wherein said function is an approximation of a Laplacian ofa Gaussian function.
 56. A system for inspecting electrical circuitsaccording to claim 54 and wherein said modifier operates downstream ofsaid convolver.
 57. A system for inspecting electrical circuitsaccording to claim 44 and also comprising: a transition locator,determining in said first image data approximate locations oftransitions between image regions having distinguishable opticalcharacteristics; and wherein said modifier is operative to removeundesired ones of said transitions.
 58. A system for inspectingelectrical circuits according to claim 44 and wherein said enhancedrepresentation is a binary representation of said electrical circuit.59. A system for inspecting electrical circuits according to claim 44and wherein said enhanced representation is a representation of contoursin said electrical circuit, which indicate approximate locations oftransitions between regions in said electrical circuit exhibitingdistinguishable optical characteristics.
 60. A system for inspectingelectrical circuits according to claim 44 and wherein said enhancedrepresentation has a spatial resolution that is greater than the spatialresolution of said first and second image data.
 61. A system forinspecting electrical circuits according to claim 60 and wherein saidenhanced representation has a gray scale whose dynamic range is reducedas compared with the dynamic range o f a gray scale of said first a ndsecond image data.
 62. A system for inspecting electrical circuitsaccording to claim 51 and also comprising: a transition locator,determining in said first image data approximate locations oftransitions between image regions having distinguishable opticalcharacteristics; and wherein said modifier is operative to override atleast part of an output of said convolver.
 63. A system for inspectingelectrical circuits according to claim 51 and wherein said enhancedrepresentation is a binary representation of said electrical circuit.64. A system for inspecting electrical circuits according to claim 51and wherein said enhanced representation is a representation of contoursin said electrical circuit, which indicate approximate locations oftransitions between regions in said electrical circuit exhibitingdistinguishable optical characteristics.
 65. A system for inspectingelectrical circuits according to claim 51 and wherein said enhancedrepresentation has a spatial resolution that is greater than the spatialresolution of said first and second image data.
 66. A system forinspecting electrical circuits according to claim 65 and wherein saidenhanced representation has a gray scale whose dynamic range is reducedas compared with the dynamic range of a gray scale of said first andsecond image data.
 67. A system for inspecting electrical circuitsaccording to claim 44 and wherein said first and second data acquisitionassemblies comprise at least one illuminator and at least one imager,comprising at least two different types of optical detectors and beingarranged to view at least a portion of said electrical circuitilluminated by said at least one illuminator.
 68. A system forinspecting electrical circuits according to claim 67 and wherein saidimager comprises three types of detectors, each of which is operative tooutput a generally spatially coincidental image of said electricalcircuit in a respective spectral range.
 69. A system for inspectingelectrical circuits comprising: a first image data acquisition assembly,obtaining first image data relating to at least part of an electricalcircuit in at least a first spectral range; a second image dataacquisition assembly obtaining second image data relating to at leastpart of an electrical circuit in at least a second spectral range; andan enhanced contrast representation generator providing an enhancedcontrast representation of the electrical circuit by non-linearlycombining said first image data and said second image data.
 70. A systemfor inspecting electrical circuits according to claim 69 and whereinsaid at least part of an electrical circuit includes first conductorslocated on a first side of an electrical circuit substrate and secondconductors located on a second side of an electrical circuit substrateand wherein said enhanced contrast representation includes informationproviding enhanced contrast between representations of said firstconductors and of said electrical circuit substrate.
 71. A system forinspecting electrical circuits according to claim 69 and wherein saidenhanced contrast representation exhibits decreased artifacts resultingfrom a non-opaque characteristic of a substrate.
 72. A system forinspecting electrical circuits formed on different surfaces of anon-opaque substrate comprising: an image data acquisition assemblyobtaining image data relating to at least part of an electrical circuit,and an image data enhancement assembly, enhancing said image data toprovide enhanced inspection output information which decreases artifactsresulting from the non-opaque characteristic of the substrate.
 73. Asystem for inspecting electrical circuits according to claim 72 whereinthe electrical circuits comprise first conductors on a first side of thesubstrate and second conductors on a second side of the substrate, andthe artifacts include images of one of said first and second conductors.74. A system for inspecting electrical circuits comprising: a firstimage data acquisition assembly, obtaining first image data relating toat least part of an electrical circuit; a second image data acquisitionassembly obtaining second image data relating to at least part of anelectrical circuit; and a pseudo-image generator non-linearly combiningsaid first image data and said second image data, said pseudo-imagegenerator being operative to supply a pseudo-image of said part of saidelectrical circuit constructed from said first and second image data toa high-sure/low-sure region classifier.
 75. A system for inspectingelectrical circuits according to claim 74 and wherein said second imagedata includes image data relating to a plurality of visuallydistinguishable substrate portions, at least some substrate portionsoverlaying conductors, and wherein said pseudo image generator redefinessubstrate portions not overlaying conductors in said second image dataas substrate portions overlaying conductors.
 76. A system for inspectingelectrical circuits according to claim 74 and wherein said second imagedata includes image data relating to a plurality of visuallydistinguishable substrate portions, at least some substrate portionsoverlaying conductors, and wherein said pseudo image generator redefinesnon-opaque substrate portions in said second image data as opaquesubstrate portions.
 77. A system for inspecting electrical circuitsaccording to claim 74 and wherein said high-sure/low-sure classifieroperates on the pseudo image to produce a high-sure/low-sure imageoutput including at least three regions: (i) a low-sure region that to ahigh degree of confidence represents only substrate; (ii) a high-sureregion that to a high degree of confidence represents only conductorlocated on the top surface of said electrical circuit; and (iii) a thirdregion which is neither high-sure nor low-sure.
 78. A system forinspecting electrical circuits according to claim 77 further comprising:a representation generator receiving said first image data, saidrepresentation generator including an override circuit in communicationwith said high-sure/low-sure classifier and operative to employ saidhigh sure/low sure image to selectively modify image data beingprocessed in said representation generator to produce an enhancedrepresentation of said electrical circuit.
 79. A system for inspectingelectrical circuit according to claim 78 and wherein said representationgenerator is operative to process said first image data.
 80. A systemfor inspecting electrical circuits according to claim 79 furthercomprising: a convolver operative to convolve said first image data witha mathematical function approximating a 2-dimensional Laplacian of aGaussian function.
 81. A system for inspecting electrical circuitsaccording to claim 80 further comprising: a transition locator operativeto determine in said first image data approximate locations oftransitions between image regions having distinguishable opticalcharacteristics.
 82. A system for inspecting electrical circuitsaccording to claim 78 and wherein said enhanced representation is abinary representation of said electrical circuit.
 83. A system forinspecting electrical circuits according to claim 78 and wherein saidenhanced representation is a representation of contours in saidelectrical circuit, which indicate approximate locations of transitionsbetween regions in said electrical circuit exhibiting distinguishableoptical characteristics.
 84. A system for inspecting electrical circuitsaccording to claim 83 and wherein said transitions between regions insaid electrical circuit exhibiting distinguishable opticalcharacteristics include transitions between substrate and conductorslocated on a top surface of said electrical circuit, and generallyexclude transitions between substrate and other conductors in saidelectrical circuit.
 85. A system for inspecting electrical circuitsaccording to claim 78 further comprising: a defect processor receivingsaid enhanced representation and being operative to analyze saidenhanced representation to provide an indication of defects in saidelectrical circuit.
 86. A system for inspecting electrical circuitsaccording to claim 74 and wherein said first and second data acquisitionassemblies comprise at least one illuminator and at least one imager,comprising at least two different types of optical detectors and beingarranged to view at least a portion of said electrical circuitilluminated by said at least one illuminator.
 87. A system forinspecting electrical circuits according to claim 86 and wherein saidimager comprises three types of detectors, each of which is operative tooutput a generally spatially coincidental image of said electricalcircuit in a respective spectral range.